Non-destructive condition assessment of insulating materials in power transformers

Abstract

Rapid in-situ analysis of the condition of power transformer winding insulation to provide information on its aged condition and water content is important for both transformer manufacturers and also asset managers in power utilities. The ability to achieve this with portable instrumentation would be very beneficial. The insulation paper degrades over time, and the degree of polymerisation of the paper affects its structural integrity and its capacity for water absorption. The use of spectroscopy together with multivariate statistical analysis provides a powerful non-destructive tool for the condition assessment of these materials both in the laboratory and in the field. We have developed a simple, portable system (TRANSPEC) using fibre-optics and broad-band spectroscopy that can measure the degree of polymerisation in the range from 100 to 1200, water content from 0.4% to almost full saturation to an accuracy of 0.1%, and oil content of new and aged transformer paper also to high precision. Multivariate statistical analysis regression models have been constructed from spectral library data and these models are used to predict the parameters above and also other physical properties e.g. tensile strength. We have also confirmed that both the water absorption kinetics and water capacity of the paper are strongly affected by the degree of degradation. With a single TRANSPEC system, in-situ analysis of the degree of polymerisation of insulating paper, the water content and, if required, a chemical analysis of the insulating oil and its water content.